<p>The growing emphasis on sustainable construction materials has prompted extensive research on agricultural and industrial by-products as a partial replacement for traditional cement. This study examines the effect of rice husk ash on the mechanical and durability characteristics of M40 grade concrete. The aim is to determine the optimum rice husk ash content that improves strength, durability, and cost-efficiency. Concrete mixes were prepared with 0%, 5%, 7.5%, 10%, 12.5%, and 15% rice husk ash by weight of cement and evaluated for compressive, tensile, and flexural strength at 7 and 28 days. Durability was assessed through water absorption and acid resistance tests. 7.5% rice husk ash replacement showed the highest performance, with an increase of almost 12% in 28-day compressive strength and improved resistance to water and acid exposure. The setting time remained within satisfactory limits, and a 4.35% reduction in material cost was achieved. The study concludes that 7.5% rice husk ash is the optimal dosage for enhancing strength and durability while endorsing sustainability. Future investigations can explore the long-term performance of rice husk ash concrete in different environmental situations and its synergy with other waste materials.</p>